1. Field of the Invention
The present invention relates to a power transferring control circuit. In particular, this invention relates to a constant voltage to constant current transferring control circuit.
2. Description of the Related Art
The constant voltage to constant current control is applied to the charging module of the lithium battery, and the current-limiting and voltage-regulating module, etc.
The charging module of the lithium battery utilizes the constant current control to quickly charge the lithium battery within the constant current control period. When the voltage of the lithium battery reaches a default protection level, the charging module of the lithium battery is switched to the constant voltage control for clamping the voltage level of the lithium battery. Thereby, the lithium battery is charged under a safety condition.
The current-limiting and voltage-regulating module uses the constant voltage control to control an output voltage. When an output current reaches to a default protection value, the current-limiting and voltage-regulating module is switched to the constant current control for clamping the output current. Thereby, the output current is limited against over-current condition.
FIG. 1 is a circuit diagram of the prior art that uses two error amplifiers to execute the voltage control and the current control. The circuit 10 uses an error amplifier 101 to receive an output voltage feedback signal VOS of the power converting module and a reference voltage level VR to perform the voltage feedback control. Thereby, the constant voltage control is implemented at the power converting module. The circuit 10 also uses an error amplifier 102 to receive the output current feedback signal VCSP of the power converting module and a reference current level VCSN to perform the current feedback control. Thereby, the constant current control is implemented at the power converting module. The control competition prevention at the output terminals of the error amplifiers 101, 102 is implemented by diodes 103, 104 to output control signal VFB to control the power converting module. The drawback of the circuit 10 is that the circuit 10 needs both the voltage feedback loop and the current feedback loop compensations. The compensation circuit adds and so the module cost increases.
FIG. 2 is a circuit diagram of another prior art that uses two error amplifiers to control the voltage and the current. The circuit 20 uses an error amplifier 201 to receive the output voltage feedback signal VOS of the power converting module and a reference voltage level VR to perform the voltage feedback control. Thereby, the constant voltage control is implemented at the power converting module. The circuit 20 also uses an error amplifier 202 to receive the output current feedback signal VCSP of the power converting module and a reference current level VCSN to perform the current feedback control. Thereby, the constant current control is implemented at the power converting module. The control competition prevention at the output terminals of the error amplifiers 201, 202 is implemented by an error amplifier 203 to output control signal VFB to control the power converting module. The drawback of the circuit 20 is that the gain of the error amplifier 203 cannot be exactly controlled. Therefore, the control loop cannot be exactly designed.